1. Field of the Invention
This invention relates to a microfilm scanning apparatus, of the entrance scanner type which term is also intended to include within its scope a so-called plotting apparatus which modulates a writing beam as it is scanned across a light sensitive film.
2. Description of the Related Art
It is known that for the ease of handling and storage of microfilm that the microfilm is mounted in a card and that apertures are provided in the card in accordance with a predetermined code to facilitate retrieval of the microfilm in an automated file handling system. So as to view such a so-called microfilm aperture card, it is known to provide a laser beam scanning apparatus comprising a laser beam diode directing radiant energy to a bevelled edge (pyramid type) polygon mirror which is rotated at high speed by a motor. The polygon mirror has a plurality of flat face facets which redirect the laser beam through a flat field lens unit to focus the beam on the microfilm, the microfilm being located on a flat platten which is driven by a stepper motor. Thus, by rotating the polygonal mirror the microfilm is scanned by the laser beam in one direction and the stepper motor drives the microfilm orthogonally to the direction of laser beam scanning so that the microfilm surface is scanned in two mutually orthogonal directions. For example, the laser beam scans the microfilm in the X direction and the stepper motor moves the microfilm in the Y direction.
However, because the microfilm is located on a flat platten, an expensive flat field lens unit is required to correct the focus of the laser beam in the direction of scanning (that is in the aforementioned X direction) since the beam is scanned in a curved path onto a flat surface.
The provision of such a flat field lens, which is expensive to manufacture, leads to a complex and expensive scanning apparatus and it is an object of this invention to provide a microfilm scanning apparatus having improved optical features which is less expensive to produce.